Require GD for saving images in xls

GD is implicitly required for saving images anyway, this exception when
it's not loaded simply gives more information on what to do if the
execution flow reaches a places that depends on a function from GD.

src/PhpSpreadsheet/Writer/Xls.php

@@ -454,6 +454,9 @@ class Xls extends BaseWriter implements IWriter
         // the BSE's (all the images)
         foreach ($this->spreadsheet->getAllsheets() as $sheet) {
             foreach ($sheet->getDrawingCollection() as $drawing) {
+                if (!extension_loaded('gd')) {
+                    throw new \RuntimeException('Saving images in xls requires gd extension');
+                }
                 if ($drawing instanceof \PhpOffice\PhpSpreadsheet\Worksheet\Drawing) {
                     $filename = $drawing->getPath();